where is <uri>model://camera</uri> ?

edit

I think there are a few things which need to be clarified.

Change name inside the include tag

The uri as you put in your question is used inside the <include> tag. If all you want is to have two cameras with different names, you can add a <name> tag inside the include as follows:

<include>
  <name>camera_1</name>
  <pose>1 0 0 0 0 0</pose>
  <uri>model://camera</uri>
</include>
<include>
  <name>camera_2</name>
  <pose>2 0 0 0 0 0</pose>
  <uri>model://camera</uri>
</include>

model://

URIs beginning with model:// look for models in paths indicated by the environment variable GAZEBO_MODEL_PATH. See this tutorial for more information.

In case the model can't be found locally, Gazebo looks for it on the online model database. Here's the camera model in the database. Once Gazebo downloads it the first time, it places the model under the ~/.gazebo/models/ directory so it can be found locally in the future. If you check that directory in your computer, you should find the camera there.